1. Field of the Invention
The present invention relates to a hot dipping apparatus employed in a manufacturing line for manufacturing hot-dip zinc-coated steel sheets or the like.
2. Description of the Related Art
A hot dipping apparatus for manufacturing hot-dip zinc-coated steel sheets carries out a pretreatment process, such as a continuous heating process for heating steel sheets or a continuous annealing process for annealing steel sheets. A steel sheet unwound from a steel sheet coil is passed through a heating furnace and a zinc melting furnace successively for continuous processing. The zinc melting furnace is replenished intermittently with zinc ingots of a room temperature as zinc molten therein is consumed. Zinc ingots to be supplied into the zinc melting furnace are carried to a place near the zinc melting furnace by an ingot carrying device. It is preferable to maintain the molten zinc melted in the zinc melting furnace at temperatures in the range of a predetermined temperature plus and minus 2xc2x0 C. to maintain constant zinc plating quality. An exhaust gas discharged from the heating furnace is discharged into the atmosphere after being used for preheating combustion air to be supplied into the heating furnace by a heat exchanger.
In the conventional hot dipping apparatus, a zinc ingot of about 1 t having a room temperature, which is far lower than the melting point (460xc2x0 C.) of zinc, is supplied into the zinc melting furnace. If a zinc ingot of 1 t is dumped at a time into the zinc melting furnace, the temperature of molten zinc contained in the zinc melting furnace drops and zinc plating quality becomes unstable. A zinc ingot may be gradually immersed in molten zinc contained in the zinc melting furnace by using a hoist to prevent zinc plating quality from becoming unstable, which, however, requires skill and labor. It is not easy to maintain the temperature of molten zinc within the predetermined temperature range. It is possible to use an ingot supply device that grips a zinc ingot and immerses the zinc ingot gradually and automatically in the molten zinc, which, however, makes system configuration complicated. The exhaust gas discharged from the heating furnace still has a high temperature in the range of 350 to 450xc2x0 C. after the same has been used for heating combustion air to be supplied into the heating furnace by the heat exchanger. Therefore, the discharge of the high-temperature exhaust gas into the atmosphere reduces the thermal efficiency of the hot dipping apparatus.
The present invention has been made in view of the foregoing circumstances and it is therefore an object of the present invention to provide a hot dipping apparatus of simple construction capable of preventing the unstable variation of a molten plating material and of operating at an improved thermal efficiency.
According to the present invention, the hot dipping apparatus includes: a heating furnace for heating or annealing a workpiece to be plated; a melting furnace for melting an ingot of a plating material and containing a molten plating material in which the workpiece is immersed to be coated with the molten plating material for plating; an ingot carrying device for carrying the ingot to a place near the melting furnace; an ingot feed device for feeding the ingot carried by the ingot carrying device into the melting furnace; an ingot preheating furnace mounted so as to enclose the ingot carrying device for preheating the ingot; and an exhaust gas line for supplying an exhaust gas discharged from the heating furnace to the ingot preheating furnace. The exhaust gas to be supplied into the ingot preheating furnace has a temperature below a melting point of the ingot.
In the hot dipping apparatus, the exhaust gas discharged from the heating furnace is supplied into the ingot preheating furnace enclosing the ingot carrying device to preheat the ingot at a temperature below the melting point thereof. Therefore, the hot dipping apparatus operates at a high thermal efficiency and the excessive drop of the temperature of the molten plating material contained in the melting furnace can be prevented when the ingot is fed into the melting furnace.
Preferably, the hot dipping apparatus further includes a temperature controller for maintaining the temperature of the exhaust gas to be supplied into the ingot preheating furnace below the melting point of the ingot.
Since the temperature controller maintains the temperature of the exhaust gas to be supplied into the ingot preheating furnace automatically below the melting point of the ingot, the ingot heated surely at a temperature below the melting point of the ingot can be carried to the place near the melting furnace and hence the ingot will not melt even if the temperature of the exhaust gas discharged from the heating furnace exceeds the melting point of the ingot.
Preferably, the temperature controller has a temperature sensor for measuring the temperature of the exhaust gas to be supplied into the ingot preheating furnace, and a control valve which is opened when the temperature of the exhaust gas measured by the temperature sensor exceeds a set temperature to supply an atmosphere into the exhaust gas supply line to maintain the temperature of the exhaust gas to be supplied into the ingot preheating furnace below the melting point of the zinc ingot.
Preferably, the dipping apparatus further includes a heat exchanger disposed in the exhaust gas line for heating a combustion air to be supplied into the heating furnace by the exhaust gas discharged from the heating furnace.
Preferably, the ingot carrying device is of a pusher system having a pusher for pushing a row of the ingots.
Preferably, the ingot carrying device is of a walking beam system having a stationary beam and a movable beam for advancing the ingot stepwise.
Preferably, the ingot carrying device is of a conveyor system having a belt conveyor for conveying the ingot.
The ingot carrying devices are simple in construction and are capable of being easily enclosed by the ingot preheating furnace and of automatically carrying the ingot to the place near the melting furnace.
Preferably, the ingot preheating furnace has an inlet which can be closed and an outlet which can be closed.
Preferably, the plating material is a zinc and the workpiece to be plated is a steel sheet.